1. Field of the Invention
This invention relates generally to accumulators for high pressure applications, and more particularly to high pressure accumulators of the piston-in-sleeve (or “piston and sleeve”) type. This invention further relates to the potential use of such accumulators in conjunction with fuel efficient hydraulic hybrid motor vehicles.
2. Description of the Related Art
1. Hydraulic Hybrid Vehicles
Hybrid powertrains are an increasingly popular approach to improving the fuel utilization of motor vehicles. “Hybrid” refers to the combination of one or more conventional internal combustion engines with a secondary power system. The secondary power system typically serves the functions of receiving and storing excess energy produced by the engine and energy recovered from braking events, and redelivering this energy to supplement the engine when necessary. This frees the internal combustion engine to operate more efficiently, while the secondary power system acts in concert with it to make sure that enough power is available to meet road load demands and any excess power is stored for later use.
Several forms of secondary power systems are known in the art. For example, various electrical systems, such as batteries and electric motor/generators, are becoming popular in commercial applications. Recently, however, hydraulic power systems have been demonstrated to have comparable or even better efficiency, while offering potential advantages in power density, cost, and service life. A hydraulic power system takes the form of one or more hydraulic accumulators for energy storage and one or more hydraulic pumps, motors, or pump/motors for power transmission. Hydraulic accumulators operate on the principle that energy may be stored by compressing a gas. An accumulator's pressure vessel contains a captive charge of gas, typically nitrogen, which becomes compressed as a hydraulic pump pumps liquid into the vessel. The liquid thereby becomes pressurized and when released may be used to drive a hydraulic motor. A hydraulic accumulator thus utilizes two distinct working media, one a compressible gas and the other a relatively incompressible liquid. Throughout this document, the term “gas” shall refer to the gaseous medium and the term “fluid” shall refer to the liquid working medium, as is customary in the art.
U.S. Pat. No. 5,495,912 (“Hybrid Powertrain Vehicle”) and U.S. Pat. No. 6,719,080 (“Hydraulic Hybrid Vehicle”) provide additional context on the role of hydraulic accumulators in hydraulic hybrid powertrains, as well as detailed descriptions on the preferred series and parallel hydraulic hybrid motor vehicle configurations for use with the accumulator of the present invention, and are therefore incorporated herein by reference in their entirety.
2. High Pressure Accumulator Design Considerations—Permeation
Care must be taken in the design of a high pressure accumulator to minimize the extent to which the gaseous medium mixes with and dissolves within the hydraulic fluid. The dissolution of gas molecules in hydraulic fluid may cause several significant problems, particularly in a hydraulic hybrid vehicle application. For example, when highly pressurized fluid is discharged from the accumulator to drive a hydraulic motor, the fluid pressure will drop rapidly and dramatically (e.g., from 5000 psi to 100 psi in less than one second) as the fluid flows through the motor. Such a pressure drop causes any dissolved gas present in the fluid to immediately come out of solution and form small bubbles or gas pockets. As the depressurized fluid is discharged from the motor, the gas travels with it, generally into a low pressure accumulator (or reservoir) where the fluid is stored until needed again. A significant quantity of gas may thus over time become trapped in the low pressure accumulator. Although it is possible to develop means to vent the gas after it collects at the fluid surface in the low pressure accumulator, the loss of this gas would nevertheless also represent a gradual depletion of the gas pre-charge of the high pressure accumulator and thus would lead to the need for occasional gas recharging (which would not be a desirable result for consumers, particularly for use in a motor vehicle application). In addition, such gas in the low pressure accumulator may also become entrained as bubbles or gas pockets in fluid that is later pumped back out, causing the pump to experience potentially damaging effects such as cavitation or torque fluctuations. Further, the accumulating volume of gas reduces the effective fluid capacity of the low pressure accumulator.
Systems that operate at very high pressures are particularly vulnerable to all of these difficulties because high pressures encourage greater dissolution of gas in the fluid, and the greater degree of pressure drop increases the rate at which gas comes out of solution.
Prior art high pressure accumulator designs therefore go to great lengths to minimize gas dissolution and ensure physical separation of the charge gas from the fluid. Several separation means are known, including the use of an elastic bladder or diaphragm, a flexible bellows, or a piston in a cylinder. However, thus far prior art separation means do not minimize gas dissolution to the degree that would be preferred in a commercially acceptable hydraulic hybrid vehicle. Such an application calls for a high pressure system that is preferably “closed,” that is, capable of operating indefinitely without frequent adjustment of gas or fluid levels in the accumulator.
3. High Pressure Accumulator Designs in the Present State of the Art
In the present state of the art, most commonly available accumulators employ an elastic bladder. Although the pressure in the hydraulic fluid on one side of the bladder is generally the same as the pressure of the compressed gas on the other side of the bladder, molecules of gas tend to permeate through the bladder and dissolve in the fluid, seeking an equilibrium concentration. Some elastic bladder materials have properties that minimize permeation, but due to the molecular nature of elastomers, permeation cannot be eliminated completely. In addition, permeation resistant, flexible coatings such as poly-vinyl alcohol can be, used on the gas side of the bladder, but even with such coatings the permeation level is still unacceptable at high pressures.
Some success has been achieved by replacing the elastic bladder with a flexible metallic or metallic-coated bellows structure. For example, the inventions disclosed in U.S. Pat. No. 5,771,936 granted to Sasaki et al. (1998) and U.S. Pat. No. 6,478,051 to Drumm et al. (2002) depict such a bellows. However, a principal shortcoming of this approach lies in the potential for the bellows to experience stresses and longitudinal disorientation that would soon lead to failure under a severe duty cycle, such as would be present in an automotive power system application.
Standard piston accumulators are also well represented in the art. In a standard piston accumulator, the hydraulic fluid is separated from the compressed gas by means of a piston which seals against the inner walls of a cylindrical pressure vessel and is free to move longitudinally as fluid enters and leaves and the gas compresses and expands. Because the piston does not need to be flexible, it may be made of a gas impermeable material such as steel. However, the interface between the piston and the inner wall of the cylinder must be controlled tightly to ensure a good seal, and the degree of dimensional tolerance necessary to ensure a good seal increases the cost of manufacturing. It also requires that the pressure vessel be extremely rigid and resistant to expansion near its center when pressurized, which would otherwise defeat the seal by widening the distance between the piston and cylinder wall. This has eliminated the consideration of composite materials for high pressure piston accumulator vessels, as composite materials tend to expand significantly under pressure (e.g., about 1/10 of an inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure). Furthermore, the need to assemble the cylinder with a piston inside traditionally requires that the cylinder have at least one removable end cap for use in assembly and repair, rather than the integral rounded ends that are more structurally desirable in efficiently meeting pressure containment demands. Composite pressure vessels are not constructed effectively with removable end caps.
As a result of the foregoing, standard piston accumulator vessels tend to be made of thick, high strength steel and are very heavy. Standard piston accumulators have a much higher weight to energy storage ratio than either steel or composite bladder accumulators, which makes them undesirable for mobile vehicular applications (as such increased weight would, for example, reduce fuel economy for the vehicle). More specifically, piston accumulators for the same capacity (i.e., size) and pressure rating are many times heavier (e.g., by up to 10 times) than an accumulator with a lightweight composite pressure vessel design, as would be preferred in such applications where accumulator weight is an issue. Therefore, despite their potentially superior gas impermeability, piston accumulators are largely impractical for vehicular applications.
4. Prior Art Regarding Piston-in-Sleeve Accumulator Designs
One piston accumulator concept, considered, for example, for military aircraft applications, utilizes a piston and sleeve assembly, in which the piston resides within and seals against a cylindrical sleeve that is separate from the inner wall of the pressure vessel. This piston-in-sleeve approach provides at least two benefits over the prior art for high pressure accumulators, namely (i) separating the pressure containment function of the vessel wall from its piston sealing function, allowing an effective seal to be pursued with a sleeve independently of issues relating to pressure vessel construction, and (ii) providing an intervening (aka “interstitial”) volume between the sleeve and vessel wall which may be filled with the charge gas to provide a safety factor against explosion in the event of puncture of the pressure vessel (e.g., by a bullet in military combat). U.S. Pat. No. 2,417,873 (Huber 1947), U.S. Pat. No. 2,703,108 (McQuistion 1955), U.S. Pat. No. Re24,223 (Ford 1956), and later U.S. Pat. No. 4,714,094 (Tovagliaro 1987) each teach the use of a piston and sleeve assembly on a high pressure accumulator. Such designs comprise a generally thickwalled strong cylindrical pressure vessel constructed of a steel alloy, and a metal sleeve which is thin relative to the vessel walls. The sleeve is permanently attached to the inner surface of one end of the pressure vessel near its circumference, creating (with the piston) a closed or “inside” chamber for the working fluid. The other end of the sleeve extends toward the other end of the vessel and is generally left open to create an “outside” chamber that consists of the open volume of the sleeve, the remaining volume of the pressure vessel, and the intervening/interstitial space between the outer wall of the sleeve and the inner wall of the pressure vessel, each filled with the gaseous medium of the accumulator.
In operation of these prior art piston-in-sleeve accumulator designs, the sleeve must be tightly retained and centered within the vessel to prevent radial movement, for example, due to vibrations in use with mobile (e.g. aircraft) applications. Sleeve movement would fatigue the rigid fixed end of the sleeve possibly leading to leakage due to cracking, distortion, or wear of the sealing gasket if one is present. This requires the sleeve to either be stiffened by connecting it at points to the vessel wall, or requires the sleeve to be thicker than the minimum that would be necessary to withstand the small pressure differentials normally encountered in charging and discharging. Further, the outer walls of the vessel must be thicker than would be necessary for pressure containment alone because the walls must be prevented from expanding and thus loosening the sleeve or distorting it from the true circular form necessary for piston sealing.
Prior art piston-in-sleeve designs also uniformly contain the fluid within the closed (inside) chamber, with the charge gas residing on the other side of the piston and in the interstitial space between the sleeve and vessel wall. This fluid-inside, gas-outside arrangement is used in the prior art for at least two reasons. First, as mentioned above, the prior inventors sought a resistance to structural splitting if a bullet or shrapnel were to enter the fluid side during military combat. With the charge gas residing in the interstitial space at the cylindrical periphery of the vessel, the displacement caused by the bullet would be largely absorbed by compression of the gas in this space. Second, this arrangement is naturally preferable because it maximizes the fluid capacity and hence energy capacity of the device. That is, the working medium that resides inside the sleeve may be discharged completely, while some portion of the medium outside the sleeve will always remain trapped in the interstitial space; because working capacity is determined by how much fluid may be discharged, it is a natural choice to have the fluid reside on the inside of the sleeve and gas on the outside.
Like standard piston accumulators discussed above, these prior art piston-in-sleeve accumulators are unacceptably heavy for a hydraulic hybrid motor vehicle application or other application where accumulator weight is a significant issue. Notably, U.S. Pat. No. 4,714,094 (Tovagliaro 1987) attempts to reduce the weight of such piston-in-sleeve accumulators through the use of lightweight composite materials in place of steel for the pressure containment function in the vessel wall. However, the Tovagliaro device still requires an internal metallic core to the vessel wall (in addition to the composite envelope, likely at least in part to resist permeation of the gas under pressure out through the composite vessel wall) and a thickened metal area at one (flat) end of the accumulator (to enable providing a removable end cap and to tightly retain and center the sleeve, as discussed above). As such, the Tovagliaro device would still remain undesirably heavy for a hydraulic hybrid motor vehicle application, and would also entail significantly greater manufacturing cost than desired (e.g., because of complexity of the design and entailing vessel construction with both a composite envelope and metallic core and end). In addition, the internal metallic core (or liner) used in conjunction with composite materials would be unacceptable for use in hydraulic hybrid vehicles. The intense duty cycle experienced by the accumulator (i.e., the extremely large number of charge-discharge cycles, in some cases exceeding one million cycles) and the significant radial expansion of composite materials (about 1/10 of one inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure) together would result in expected fatigue failure of the metal core or liner.
In addition, the flat end construction (on at least one end) of prior art piston accumulators also adds significantly to the complexity, weight and cost of the accumulator.
5. Disadvantages of the Prior Art
In summary, as has been explained above, despite the many years of development for accumulator designs, the prior art has thus far failed to provide a high pressure accumulator design that is lightweight, low cost, durable under stresses, and does not have permeation difficulties at high fluid pressures. Prior art bladder accumulators have unacceptable permeation. Prior art metal bellows accumulators are not sufficiently durable under stresses. Prior art piston accumulators of all types are unacceptably heavy and costly. As a result, the prior art has failed to provide a high pressure accumulator that is satisfactory for hydraulic hybrid motor vehicle applications, as is desired for the present invention.